This disclosure relates to a fault current limiting system and method, and in particular relates to a system and method for high-speed short-circuit current interruption.
Bus-tie components which are able to interrupt very high short-circuit current in a time period which is a fraction of a period of the current are known in the art.
The goal of conventional fault current limiting components is to allow downsizing of both left and right bus-bar systems, in comparison with the total short circuit current supplying a feeder, as represented in FIG. 1.
The downsizing is reached by dimensioning each side of the switchboard for only the short circuit power of its relevant in-coming feeder. This downsizing is possible only when a very fast interrupting device is located in the bus-tie position. With reference to FIG. 1, in the case of a short circuit condition (4) of the out-going feeder (1), e.g. in the right hand side of the switchboard in FIG. 1, the very fast interrupting device should be able to avoid any contribution to short circuit peak current coming from the other half (2) of the switchboard not affected by fault conditions, i.e., the left hand side of the switchboard in FIG. 1.
This downsizing is extremely interesting in economic terms for a new electrical installation, where all the new equipment can be purchased for a fraction of the total short circuit power supplying the complete installation plant, with corresponding relevant savings. Cost savings can also be achieved in case of installation up-grade, when an existing plant has to be adapted to an increase demand, thus avoiding the necessity for upgrading and changing all existing apparatus.
The above-described solution can be easily implemented by simply using a fuse in the bus-tie compartment. However, fuse technology has certain limitations, and it is not always possible to have fuses with a satisfactory nominal current. Therefore, in conventional practice, the use of a fuse in the bus-tie compartment is only effective for very small installations.
The solution for bigger installations foresees the use of a fuse, which is supplied only by the short circuit current, while the nominal current is normally let through a low resistance parallel element.
This parallel element may be an explosive cartridge, which is detonated at the moment the short circuit condition is detected. A control device takes care of detecting this short circuit, by both measuring the current amplitude and its rate of rise.
The present state of the art, although satisfactory for the basic needs, has some disadvantages.
At the end of the short circuit intervention, the whole system, including both the explosive cartridge and the fuse, needs to be replaced before allowing the distribution system to recover complete functionality. This requires a maintenance operation, which takes time, and requires non-conventional spare parts, such as the explosive cartridge. During this maintenance time, the system is operating on both sides but not with the full short circuit power available, i.e., no loads are disconnected, but some operating condition may not be possible due to voltage drop and start-up limitations.
Also, all the working functionality is based on an explosive device which, in some countries, can create difficulties in transportation and/or property permissions, or in others, simply be banned.
A further disadvantage is given by the fact that the explosive-cartridge requires an electronic control that is separated from the switchboard control system. This requires, in practical terms, to have at least two different electronic control devices inside the switchboard, with all difficulties related to this.